1. Field of the Invention
This invention relates to an automatic control of armour tape tension, particularly when wrapping metal tape around electrical cables for mechanical protection. More particularly, the invention relates to a method and a system for continuously and automatically controlling such armour tape tension during the wrapping operation.
2. Description of the Prior Art
Wrapping of tapes around continuously moving objects, in particular the wrapping of metal tapes around electrical cables is accomplished by continuous armouring machines, such as those known as "BX" machines, which are designed to wrap the tape around the cable from a reel on a spindle which rotates around the cable at high rpm (e.g. in the range of 1000 rpm).
The machine is provided with tape feed rolls which must be set up so that the tape forms a tube around the cable which neither cuts the cable nor allows it to be too loose within the armour. To control this tube, the machine uses a P.I.V. variable speed drive (P.I.V. standing for "PROPORTIONAL INFINITELY VARIABLE" transmission), which pushes the tape out by means of the feed rolls to be wrapped around the cable. Such P.I.V. drives are available on the market; for example, there is one sold under the trademark LINK BELT.RTM. by FMC Corporation. By adjusting the ratio of push to rotational speed, the size of the tube can be controlled. Instead of the P.I.V., two motors with two
drives can also be used.
Presently, to control the size or tightness of the tube, the operator manually adjusts the P.I.V. using a hand wheel or the speed control of drives on the motors using a potentiometer. These control methods, however, are unsatisfactory since once the run has started, the operator cannot check the tightness of the armour on the cable and, as a result, cables are often damaged by tight armour or rejected for loose armour.
It should be mentioned, in this regard, that there are fairly strict UL (Underwriters Laboratories) standards for metal-clad cables, which require that "Interlocked, corrugated, or smooth armour on a cable containing any No. 4 AWG or larger insulated conductor(s) shall grip the cable to keep the conductor assembly from being withdrawn from a sample 10 ft or 3 m long by the application of a pull of 30 lbf or 133N or 13.6 kgf". At the same time, of course, the tape should not be so tightly wrapped as to cut into the cable.